The present invention relates to a multilayer system comprising a base layer consisting of a thermoplastic, a primer layer, and a scratch resistant layer consisting of a scratch resistant top coat, the multilayer system being characterised in that the primer layer contains a certain UV absorber and a certain amount of acid, preferably acetic acid, is added to the scratch resistant top coat.
Owing to its good optical, mechanical and thermal properties and because of its relatively low weight, polycarbonate is suitable as an alternative material to glass and as a material for use in glazing applications. Its relatively low scratch resistance and weathering resistance are disadvantageous, however. These drawbacks are often compensated for by the application of coatings containing UV absorbers.
Biphenyl-substituted triazines (WO 2006/108520 A) are a very good class of UV absorbers according to the current prior art. This class of substances has an outstanding absorption in the UV range of 320 to 380 nm and also a very high inherent UV stability (WO 2000/066675 A1, U.S. Pat. No. 6,225,384). Owing to their very pronounced aromatic nature, most substances of this class known to date dissolve well only in non-polar and moderately polar media. The known commercially available triazines can be used in UV-curing coatings formulated on a purely organic basis. However, these kind of coatings are not adequate for applications requiring a relatively high degree of scratch resistance.
If the material is to be effectively protected against abrasion and scratches, sol-gel silicate coatings (see e.g. EP-A 0 339 257, U.S. Pat. No. 5,041,313) and other hybrid coatings (EP-A 0 570 165) are the current prior art. The properties of organosilane-based coatings include, in addition to excellent weathering and light stability, resistance to heat, alkalis, solvents and moisture. However, as non-polar additives are not soluble in these coating systems, commercially available moderately polar, mostly hydroxy-containing UV absorbers and/or inorganic UV absorbers, such as titanium dioxide, zinc oxide or cerium dioxide, are used as UV absorbers, although their action leaves room for improvement (EP-A 0 931 820). DE 10200901943.2 describes how the polarity of UV absorbers having a triazine structure can also be improved by modification with trialkoxysilane groups. The objective of this application is to provide triazine derivatives which dissolve in the polar scratch resistant top coat.
Alternatively or additionally, non-polar or moderately polar UV absorbers can also be dissolved in a primer layer which is applied underneath the scratch resistant layer. The primer layer contains organic binder materials which have good adhesion both to thermoplastic surfaces and to the scratch resistant top coat. Primer layers for adhesion improvement between thermoplastic base layers and siloxane-based scratch resistant top coat are known for example from U.S. Pat. No. 5,041,313 and U.S. Pat. No. 5,391,795 and from the patents cited therein and are used as adhesion promoter in the coating of polycarbonate. In addition to promoting, these primer layers can also contain a UV absorber as mentioned in U.S. Pat. No. 5,041,313 and U.S. Pat. No. 5,391,795, thereby contributing to the weathering resistance of the coated part.
A layer structure consisting of polycarbonate, primer and top coat is known from EP-A 6 727 32 in which the primer layer contains a UV stabiliser based on resorcinol derivatives.
It is also known from WO 2009/049904 A1 that primer layers containing triazine-based UV absorbers of formula (V):
in whichX═OR6, OCH2CH2OR6, OCH2CH(OH)CH2OR6 or OCH(R7)COOR8, in whichR6=branched or unbranched C1-C1 alkyl, C2-C20 alkenyl, C6-C12 aryl or —CO—C1-C18 alkyl,R7═H or branched or unbranched C1-C8 alkyl, andR8═C1-C12 alkyl, C2-C12 alkenyl or C5-C6 cycloalkyl,are particularly suitable and also have very good weathering resistance. The use of a combination of UV absorbers of the hydroxybenzophenone class, in particular resorcinol-based UV stabilisers with triazine-based UV stabilisers of formula (V), particularly in primer compositions for transparent thermoplastic substrates, such as in particular those consisting of polycarbonates, is also described as being particularly suitable.
The use of these triazines increases the UV absorption performance of the primer layer as compared with conventional primers in a comparable thickness. Alternatively, it is possible to lower the viscosity of the primer composition so that the thickness of the primer layer can be reduced. The minimum thickness of the UV-absorber-containing primer layer is determined by the desired UV protection, as the UV absorbers have a limited solubility in the primer composition.
If the concentration of the triazine UV absorber in the primer layer is too high, it precipitates out and the multilayer system becomes hazy.
To increase the adhesion of the scratch resistant top coat to the primer layer, it has proved advantageous to add acetic acid to it. Acetic acid in the scratch resistant-top coat leads to the formation of an interpenetration layer (IPL) between the primer and the scratch resistant top coat, which further improves the adhesion between the two layers. The scratch resistant top coat penetrates into the primer layer and a further mixed primer/scratch resistant interlayer is formed between the scratch resistant top coat layer and the primer layer. The formation of the IPL increases the long-term stability of the multilayer systems, as is demonstrated for example by reduced crazing in the scratch resistant top coat layer under weathering. The triazine compounds which were up to now used in the primer layer (see WO 2009/049904 A1) and which are suitable in particular for protecting polycarbonate, are insoluble in this IPL layer, however. Therefore when acetic acid is added to the scratch resistant top coat they concentrate in the remaining primer layer, whose thickness is reduced. As the acetic acid content in the scratch resistant top coat rises, the thickness of the IPL layer increases and hence the remaining primer layer becomes ever thinner, leading to haze caused by undissolved/precipitated UV absorber.
The objective was therefore to provide a multilayer system suitable for glazing applications which demonstrates both an effective and lasting UV protection combined with good weathering resistance and very good adhesion between the primer layer and the scratch resistant top coat arising from a thick interpenetration layer.